Hydraulic shock absorber with initial impact damping



July 13, 1965 J. R. JERoMsoN, JR 3,194,355

HYDRAULIC SHOCK ABSORBER WITH INITIAL IMPACT DAMPING Filed Oct. 2 1965 United States Patent 3,194,355 HYDRAULHC SHCK AESRRER Wi'iii IN'HAL iMPACT DAMPHNG .lames R. .leromsom Jr., Willoughby Hills, Ghio, assigner to Thompson Ramo Wooldridge Inc., Cleveland, Ohio,

a corporation of @hic i Filed Unt. 2, 1963, Ser. No. 313,254 3 Claims. (Ci. 18S- 100) This 'invention relates to hydraulic shock absorbers of the direct acting Aor telescopic type, and more particularly concerns such shock absorbers provided with means for preventing or at least significantly attenuating the sudden pressure pulses or shock waves which are often propagated as noise and/or as jolts through the shock absorbers and to the apparatus such as vehicle structures with which the shock absorbers are used.

One of the functions of the customary elastomeric bushings between the connecting eyes of direct acting shock absorbers in the knuckle connections with opposed relatively movable portions of a vehicle, for example, is to prevent transmission from the shock absorber of the relatively high frequency pulsations or shock waves resulting from the sudden pulses or thrusts of the piston relative to the cylinder of a magnitude or frequency greater than will permit, at least momentarily, displacement of the hydraulic fiuid through the valve system of the shock absorber. Such bushings in order to perform most effectively for cushioning should be relatively soft. However, these bushings must also sustain and transmit the maximum damping force encountered during both compression (or jounce) and rebound strokes of the shock absorber. For this latter purpose, therefore, and to perform without undue extrusion of the rubber in the bushings, they must be made of a harder durometer stock. The bushings are therefore actually only relatively inefficiently absorptive of the shock waves.

Moreover, the shock waves also produce noise which is commonly referred to in the automotive trade as chuckle Such noise is, of course, the audible manifestation of vibrations in the metallic parts of the shock Y absorber and which are commonly transmitted to the associated vehicle and felt by the passengers as a jarring, uncomfortable shock.

The desirability of eliminating the disadvantages and undesirable results of sudden pressure pulses or Waves has been recognized but no really efiicient means have heretofore been provided for simply and economically curing the problem. For example, in Boehm et al. Patent 2,781,869, dated February 19, 1957, it is proposed to supply the compression and rebound chambers, of a hydraulic shock absorber with rubber air sacks enclosing respective volumes of air which is compressible by collapsing of the rubbervsacks to avoid the shocks incident to the relative incompressibility of the hydraulic fluid during the initial resistance to relative movement of the piston and cylinder responsive to high magnitude or frequency shocks. However, the disadvantage of such arrangement resides in the undue softness and relatively long travel permitted in the stroke of the shock absorber before orifice damping occurs at the very functional point highest shock absorbing ordamping efficiency is most desirable. In other words, compression of the gas or air in the cushioning sacks results in a long relatively fiat compression curve instead of the usual steeply ascending curve customarily attained by orifice flow damping.

It is accordingly an important object of the present invention to attain eiicient elimination or at least significant attenuation of the sudden pressure pulse or wave reactions or shocks in the operation of a hydraulic shock absorber While maintaining the stroking characteristics of the shock absorber substantially close to the usual orifice flow damping rate.

Another object of the invention is to provide new and improved non-orifice supplementary shock and vibration damping in an essentially orifice or controlled passage iiow shock absorber.

Still another object of the invention is to provide a hydraulic shock absorber with novel smoothly transitional spring rate cushioning.

Yet another object of the invention is to provide new and improved auxiliary damping means in the hydraulic shock absorber utilizing multi-closed cell elastomer means.

A still further object of the invention is to provide in a hydraulic shock absorber new and improved combined sudden pressure pulse or wave damping and stroke end buffer means.

Other objects, features and advantages of the present invention will be readily apparent from the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawing, in which:

FIGURE l is a more or less schematic longitudinal sectional view through a direct acting hydraulic shock absorber embodying features of the invention;

FIGURE 2 is a representation of a compression stroke load curve in test stroking a standard type of direct acting hydraulic shock absorber;

FIGURE 3 is a stroking curve of the same shock absorber without any hydraulic duid therein; and

FIGURE 4 is a load curve showing the results during a test stroke under the same conditions as produced the curve of FIGURE 2, but modified with the mass of multiclosed cell elastomer in the compression chamber.

An important advantage of the present invention resides in that it is useful in suppressing shock waves and smoothing out the operation of any preferred construction of orifice ow direct acting or telescopic shock absorber, and more particularly the type of shock absorber which is now standard equipment in automobiles wherein there are generally four of such shock absorbers used as part of the undercarriage of each automobile. Shock absorbers of this type are also used in the undercarriage of airborne vehicles comprising airplanes, helicopters and the like.

For illustrative purposes, a more or less schematically illustrated shock absorber 5 is depicted in FIGURE 1 including as principal components an elongated tubular cylinder 7 having reciprocably operable therein a piston S attached to an upwardly extending concentric piston rod 9 of `smaller diameter. Within the cylinder 7 below the piston rod 2i is defined a jounce or compression chamber it@ and above the piston 8 is defined a rebound chamber lil. These chambers are normally filled with suitable hydraulic fluid such as oil, and in the relative reciprocations of the cylinder 7 and the piston 8 this oil is metered through a suitable arrangement of orifices 12 generally, though not necessarily having associated control valving, by which frictional, damping, energy absorption is attained in the operation of the shock absorber.

Concentrically assembled with lthe Ilower end of .the cylinder 7 is a foot valve structure 13 engaged concentrically with a lower closure cap member 14 to which is concentrically secured a reservoir tube or casing 15 of larger inside diameter than the outside diameter of the cylinder 7 to define a reservoir 17 into which oil displaced ,by the .piston rod 9 during compression strokes is metered yfrom the compression chamber 10 through the :foot valve assembly 13 and from which the oil is returned through -the foot valve assembly during rebound movement of the piston. Carried fixedly centrally on the lower side of the cap member 14 is a rigid connecting member, herein f comprising an eye ring itl of Van inside diameter tore-` ceive aresilient bushing 119 engaged about means suchras a bolt 20 or the like by which the lower end of thez shock casing tubeY 1li-5 engage concentrically with a .closure and bearing plug body 21 providing a bearing, for the piston .rod9,fand upon which is engaged an end cap 22 retaining Y Y the reservoir casing tube l5 and the Vvupper end cap ZZ as i packing 23 about the piston rod. P rotectively enclosing Y collapsible air iilled tubesipermit some lengthening VoftheV well as .the upperYend portion of the lpistonrod 9 is a dust 1 and gravel guard shield tube 2,4 attached tothe upper,

Y y outer extremity of Ithe piston rod adjacent to means herein comprising an attachment eye ringrZS `for securing the upper end of the piston rod-operatively tothe sprung eye 25, as also, or ei-therthe lower connecting eye l may Y 1 carriage or chassis'of the associated vehicle through the Y medium `of meanssuch as .a bolt `27'k and an interposed re- .be merely astud, as is a Well known equivalent inthis art.

Normally the springs of theV associated vehiclehold the body or chassis and `the axlehousingsin spaced sprung relation, and the pist-on 8 therefore assumes they generally* Y midway positionin thecylinder 7 asV shown.`v VAny ten- 1 t v dencyV towardexcessive'springing movements and more particularly generally vibratory movements between'the sprungist'r-uctures to which it is attached are suppressed.

by theshock absorber 5. ,Howeven since the hydraulic dampingluid or' oil kis relatively incompressible and the `inherent resiliency of the metal parts ofthe shockab-v sorber are virtually'negligible, short amplitude, high shock loads or rapid short duration high impact loads, and'particularly those ,involving high frequency vibratory,Y flutter conditions or approaching lower frequency' audible sound,

[and more .particularly during the 'compressionjstroke are :virtually undamped due to the inability of the metering orifices in the piston f8 and the foot Valvelto pass the .relativelyincompressible viscous .damping liquid there-V ;'thr-'oug'h jat the velocities Vand volu-mes demanded by the' sudden pressure pulses. or'sh-ock Waves generated bythe high lshock or impact loads.V As a result in a conventional shock absorber, such shock-waves are virtually undamped f1 'i :and the shock vibrations on impactareV transmitted Vthrough the Vpistonand piston rod and its connection with the body of thevehcle asia tactile reaction or perceptible.` .'jolt, oftenk apparent as a sonicvibratidn in the-damper system,Y commonly referred to'in the automotive trade as Heretofore often highlyY complex types of; series or parallel operable porting and v-alving and the v like have been pro-posed to solve this problem, but'have generallylbeen of such complex-ity and otentimes uncertain landlitrnited performance as to be unwarranted yfrom z a kc`0staperformance standpoint.V Therefore, a lowcost f. Y valve arrangementhas lgenerally beenused', with corn-l promise attempts at auxiliary damping by use of ad-ditional springs; various *rubber bus-hing arrangements. in

the connections, and,` the like.

One prior attempt at overcoming the -problemin 1a simple manner is disclosed in Patent No. 2,78'1,869`where-V Y Iin rubber'tubes lilled` with air are enclosedin free-heating relation within the compression and reboundcha-mbers of;

the'shock absorber to afford, as stated in thatfpatent'free I damping;mass, at leastY;a.preponderancelof Ysmalls'izeT Y Y Y' bubbles 1s desirablein theV damping massa-ggfor'lrexampk Y j from .OGlY to .03 Y inch f'diameter, VAdependiiigpupo'ru.theV l travel of the piston.V That is, pressureexerted through Y.

the liquid in the shock'absorber `during stroking tot the piston causes compression of the airirwithinthe single air wcellsof the rubber tubesrwith collapsing of the'tubesf However, the freetravel penmittedl'by this arrangement Visobjectio'n-able because the-very purpose of such a shock absorber isV to reduce the amount vof `free travel which is "normally permitted by the .vehicle springs. Y Y VWords, a shock :absorber should reduce. the amplitude of `InY .l other the springjfwave motion and hatten the kcurve rather than lengthen it.Y Thus, while theorice damping afforded byy the shock absorber Yreduces theY springingfamplitude', the Y l range ofsi'zes; :V Y'

amplitude due to :the free travel.Y 'Y

Eur-ther, the air celltubes of theaforesaidpatent not sustain .a Vblow atlthe extreme end cfa stroke where the c ell will escape-into ,the body of damping-liqL-uidiY y According to the presentj invention, lmeansz-are provided` at least Vin the'compressionichamber, but Ideisirrably;alsoV in [the rebound'chamberofthe .shock absorber 5 ,taking Y advantage ofthe compressibilitysofa,gasiandtheenergy l .absorbing frictional'characteristicsV of gas Yand elastomersv Y 'todamp the shoclrwaves'generatedor tending tobe gen-YVV erated during highimpactloads onthepiston f8.5 VY'For this j purpose,Y a resiliently'deformable, ,elastomeric VVdamping V 1, L; mass 29, a substantial partof the'volume 'cuvvhiohcOrh-"Y't prises gas-iilledysmallfuncorinected,jdiscrete'and inde-if Y Y pendentlylactin'g, generally fbubbledikecells'.Si), is dis-Y; 5;.

posed in preferably freeoatingfrelation Vinfthe .compres- Y Y sion charrillterliliV In a desirableaormfthedarnpingmassV 29 isYof fgenerallyspheri'cal oirballshapefQiWhereYthein-i i Y Vside diameterdot thefcylinder ,f'ji'slaboutV one inchythedi-f Y, Y. ameter of the ba-l-l shouldjbe `slightly:smaller topermit Airc-:er tio-ating movement of the balLftForeXample, where?) thenside diameter ofthefcylinder isrgabout LZ998`inch, the 'y Y ball may be about .975 inch indiameter; ,Y

' A nysuitable, oil resistantv elastomeric material may bef YV used in the damping mass 29 capable of. formation Ytherein and sustentation'of theymulticlosed cellstrueture. For 1. example, Vsyntheticv rubbers suchfas burla-N;'plzrnlymerizedY.IV chloroprene,-Yand the like, Vadapted .tobecuredjin a ,moljfl Y as by vulcanization vor balringarey suitable for the purpose.V A compositiouofhe selectedrubbermaterial. including' lJY a plasticizena vulcanizing 'agen-t,AV a gas-liberating agentV a single stagelblowingcure will sucezjsince' itispo'ssible thereby to attain a damping Vrnassthat has Vapreponderance of gasslled- A( nitrogen for vexample) unconnected, discrete' generallybubble-lilre Vcells witha cell Wall-nlatrx lcon-E nected throughout theV bodyof the:dampingprnassgandlaf-g fording Va highly etlcientinternalfriction `darripi'nglstr'uc Y although higher costgfatorsmustbeconsideredzY Yi .v

Because Y of :the high vshock loads to befgimpartedi Ytoj strength of the material" and,v fthe ,percentage` fof :confined i i gasin the bubblesw. Cyenerally,.itvis simplest 'tof form` t Y the` bubble'sfmore or. lessk uniformlysjthoughout.ithegfmassiv, Vexcept ata progressive :transition or boundary region Vor .Y skin that may be on the4 surface 'ofthe mass'althoughfsuch Y' a skinis not essential and in someinstancesmay befriegf Y ligiblbrnot materially greaterthauibubblejfyvall thick-lv ness. Utteruntormivty of bubble sizeris uotje'ssential since 'l :i a percentage, such :Yas'rlOor- 15 %l Vor more ,inlay .be'rlarger "Y .Y Vonsmaller'than a mean bubble sizesrange, Vso long Vlas ther.Y

I w3 Pfifed if j Y lnsteadofvulcanigation curing,vs'omeniateials suitable Y or preferred Y-and"1ending themselves vtofself-curing'from *i 'Y aV fluent state, `e.g. certain ,polyurethane ormulations, may Yf be employed;1f Y Y 'g 1; Y' ,sniconetype Vvminiers are quitey desirable because or@ their hightemperature resistance',@udg-also becausefof K p superior bubble',formingand.bubblefholding'propertiegf Y Under compression, the multi-closed cell elastomeric mass 29 reduces in volume by bulk compression as distinguished from bulk deformation. That is, instead of iioW- ing, the material compresses into itself. As such pressure-caused compression takes place, the bubbles nearest the area of impact are compressed and forced against the adjacent bubbles deeper within the mass which exert a resistance to the compressive force, and this reaction occurs progressively inwardly and results in a progressive reduction in transmitted pressures interiorly of the mass. This progressive reduction in transmitted pressures, as Well as bursting forces in respect to the individual bubbles is greater for smaller diameter or smaller volume bubbles and increases rapidly as they become smaller down to practical limits. This phenomenon may also be described as a spring rate characteristic, comprising the rate at which resistance to deformation occurs. As a practical matter, the spring rate characteristics or stiffness of the material in the elastomeric damping body 29 should be such as to avoid any appreciable free travel of the piston 8 within the pressure range that may be accommodated by damping displacement of hydraulic fluid through the damping orifice system including the orifices 12, thus attaining all of the advantages inherent in the orifice damping function of the shock absorber. Coupled with this desirable attribute, advantage is taken of the unique ability of the multi-closed cell elastomeric da-mping body Z9 to yield at an increasing spring rate under shock loads and more particularly shock load pressures transmitted thereto through the relatively incompressible hydraulic fluid in the compression chamber 10. As a result of the at least slight decrease in volume, the extent of which depends on the magnitude of the shock load, the rate of pressure rise in the hydraulic fluid remains below that value which would cause a shock wave to be generated. For this purpose, the generally spherical shape of the mass 29 affords substa-ntial advantage in that although the bulk compressibility of the mass is of an order of stiffness which will require a shock load on the order of that which would in the absence of the damping body 29 cause a shock wave to develop in the hydraulic uid in order to effect compression of the damping body by the hydraulic fiuid, there is uniform inward pressure applied by the hydraulic iiuicl over the entire spherical surface of the damping mass. In effect, therefore, the damping mass is entirely confined within the pressure applying medium, namely, the hydraulic fiuid with consequent attainment of a high spring rate by the applied shock force, all Working toward the common center of the elastomeric damping mass 29. Unusual smoothness of operation of the shock absorber is therefore assured.

|lest results have graphically demonstrated attainment of the new and improved results as outlined hereinbefore. Readings as recorded on a strip chart in a test fixture in which continuous measurement ofthe transmitted force in stroking of a standard, commercially available direct acting automobile shock absorber having the general characteristics of the shock absorber are depicted in FIGURES 2-4.

For control, comparison purposes, the shock absorber employed in the test was first stroked in the fixture without any elastomeric damping means therein, namely, in the standard or conventional operating condition of the shock absorber. The test shock absorber had a cylinder bore diameter of about .998 inch. It was stroked in a 2% inch stroke at 107 cycles per minute with a substantially sinusoidal velocity pattern, with an average velocity of approximately ten inches per second, attaining a maximum pressure thrust lof about 100 lbs. p.s.i.

In FIGURE 2 is depicted a typical stroke-load curve resulting from stroking the shock absorber under the conditions just outlined without any cushioning elastomer body, but fully operatively filled with hydraulic iiuid. The horizontal base line B represents the zero load condition during the return interval in a stroke cycle, while the load curve C represents the load conditions attained during compression stroke portion of the stroke cycle. As will be apparent, in approximately the first half of the compression stroke there was a very rapid increase in load, with a markedly rapid load increase at the beginning of the stroke, refiected by the sharp rise in the curve, represented by the reference character P, reaching a peak about midway through the compression stroke and then tapering off to drop on almost a straight line R to the zero terminus of the stroke.

ln order to eliminate any false reading due to seal friction in the shock absorber, it was tested empty of hydraulic fiuid, with the strip chart recorded results depicted in FIGURE 3. There it will be observed that load resistance is reflected for only a very short initial portion S of the load or pressure stroke, with an immediate fiattening of the curve to a short terminal portion of the drop-off side R of the curve.

In comparison with the test results as recorded in the curve patterns of FIGURES 2 and 3, the test results as recorded in the curve pattern depicted in FIGURE 4, wherein the shock absorber was not only filled with hydraulic fluid but also had an approximately 3.5795 gram, about .975 inch diameter multi-closed cell elastomeric damping ball in the compression chamber, graphically show the substantial new and improved operation of the shock absorber. After the initial seal friction resistance S, the load curve assumes asymptotic shape, demonstrating an initial phase, short duration softness or yielding during about the first quarter inch of the stroke as indicated at P. Then a smoothly progressive but rapid pressure increase is indicated at P in the load increasing portion of the stroke. This results from the initial impact yielding and compression of the multi-closed cell elastomeric damping mass 29, merging quickly and after only short stroke progression into a rapidly increasing spring rate stiiening of the ball Z9, and orifice damping functioning of the shock absorber to the top ofthe load portion P of the load curve C. It may also be noted that there is at least a slight lessening of the maximum pressure build-up, showing more rapid damping out, since the curve peak is reached before the maximum load peak in the FIGURE 2 showing of the stroking characteristics of the shock absorber without the auxiliary shock damping multi-bubble elastomer device. Also noteworthy is the fact, indicated by the greater outcurving of the load drop-off portion R of the curve in FIGURE 4, that dropotf is more gradual than the corresponding drop-off in FIGURE 2. here is thus conclusively demonstrated that at the beginning of an abrupt high load compression stroke there is a significant critical damping effect on the initial impact to the shock loads, attained by the presence of the bubble elastomer damping mass 29 in the compression chamber It) of the shock absorber, with quick, smooth transition to orifice flow damping of lower peak load and thus less strain on the system and mechanism, compared to parallel conditions without the mass 29.

Another important advantage of the bubble elastomer damping mass 29 in the compression chamber resides in that it will also effectively serve as a limit stop to prevent harsh jolts in strike throughs during compression stroke, that is contact of the piston S with the foot valve 13 or of the piston rod end connector 25 with the end cap 22. Dur-ing such extreme movements of the relatively telescopically related parts of the shock absorber, the piston and foot valve compress the damping member 29 therebetween and since the member is only slightly smaller in diameter than the bore diameter of the cylinder, it promptly squeezes into confined relation to the bore wall and then as the elastomeric member becomes confined against further spreading, it bulk compresses with a rapidly increasing spring rate and positively cushions the piston and shock absorber casing to a substantially 1 I claim asmy invention: Y

jar-free stop.v Further,Y by virtue of the 'multi-closedi cell structure of the elastomeric damping member 29; 1 it strongly resists rupturing due to such impact engagement;` a Although it is more important to, effect damping against shockwave generation'in the compression chamber 10,

compression chamber. To this end,y a multi-'closed cell elastomeric damping mass `31 Yhavingjclosed gas-filled cells 32 is provided in the rebound chamber 1,-preferably constructed as a torus or annulus or a ring-shape-'having Y A Y i said massbeing ofa ,diameter only slightlygless than 1 an Voutside diameter ,dimensioned to permit free', sliding Vof the ring body within the cylinder bore andwithjclearf,V Y

l5 kwherebyshockvvaves createdbyjgpressurejsurgesvof `a--lQ' Y' l magnitude greaterlthanfcan -be?saccommofdatejd by thea; orhicejmeans'will be absorbe'diby bulkfcompre'ssionof ff Y said mass yand continued compression movementof' the! PSOH iS reecfedinrapidly "increasingfspringrate re-V-` 2,0"sistance tocompressionf saidmassand dampingotherl Vwise proceedingV by displacement offhydrauliciiuid through; Y

ance'between its inside diameter-and the' piston rod. An

axially-elongated cross-section is preferred. Similar oil resistant materials and similar relationships of' -closed cells or bubbles to the elastomer matrix atording` similar springV rate characteristics are Vadapted to be employed in; the damping ring mass 31 asin ,th'e'damping ballgmrass 29: A During rebound strokes ofthe piston 8 sharp,:high loadY shocks aredamped by the ,damping mass 31,. similarly vas described for the dampingrmass?.V vFurthenfthe multiclosedV c'ellelastomeric damping massil serves as Va cushtoning limit stop between the piston 8 and the bearing closure member` 21.'- i Y Addition. of the free floating multiclosed1` cell elastomeric damping masses 29rand 31 to the hydraulic shock absorber requires noreconstruction of the damper structure, but merely a placing of the freefloating respective Vmasses in the respective. Chambers in they shock absorber cylinder;v It willbeunderstood that modiiications and variations Y may be eiected Without departing from the scope of the l novel `conceptsof the present invention.

i 1.A Inra direct acting hydraulic piston rod extending through said closure bearing, Vthe Y piston dividing the cylinder bore into a compression chamf l berbetween the piston and the foot'valve and a rebound f chamber between the piston and the closure bearing, both of said chambers being filled with hydraulicuid and the` piston having metering orifice t means .therethrough for ment comprising:

5 a'substantiallyspherical free'floating7 massinjsaidcflme'i i pression chamber between'said", foot valve and 4said A i Y pistonland;` in; uninterriii:tedy alignment, .Witl'rY s.id 'V orice, means,

1'said mass being of bullr;compressiblejelastomeric.ma i Y 10 e Aterial asubstantial p art of the volume'ofwhich com the'diameterf'of saidcylirider bore,

saidoricefrne'ans and saidvlfootIvalveQ/f -f 21A shock absorberfas deiined A in`Y `c laiin 1', vin Whichzf theV piston'isadaptedfto drivelsaidmassain terminal Icom- .25p ressior 1.strke'against the ioctl-valve whereby the mass Y is squeezed by thep'is'ton into1 ckoniiedrelatienfto fthe g bore 'wall andV then as` the mass `beconie's conined 'against Y Yfurther spreadingit compresses withfaj rapidly increasing; spring' ratev to positively'` rcushion fthepiston fto" afsubstanl l 4prises gas-lille'd, small;'unconnected,discretef and 'in-r 3.7,A`dir'ect acting shockabsorber- Y as vdb iined in claini.. -Y Lin which a compress'iblel elastomeric massof'gthe' same material as saidgs'pherical mass-'isdisposedinffree float-ff Ving' relation in the rebound chamber 'fordampinggshock waves'during 'reboundg andV also servingasa rebound,

bumper betweenthepiston-:andsaidjclosurebearing; A

shock absorber compris- Y Y ing a tubular cylinder having a bore'clo'sed bya foot valve at one end, means dening a reservoir contiguous V to the cylinderand with which the cylinder bore com`Vr municates through Asaid foot valve, a closure bearing clos-V in g the opposite 'end ofythe cylinder` bore, Vazpiston reciprocably operable ini the'cylinder bore and having a 

1. IN A DIRECT ACTING HYDRAULIC SHOCK ABSORBER COMPRISING A TUBULAR CYLINDER HAVING A BORE CLOSED BY A FOOT VALVE AT ONE END, MEANS DEFINING A RESERVOIR CONTIGUOUS TO THE CYLINDER AND WITH WHICH THE CYLNDER BORE COMMUNICATES THROUGH SAID FOOT VALVE, A CLOSURE BEARING CLOSING THE OPPOSITE END OF THE CYLINDER BORE, A PISTON RECIPROCABLY OPERABLE IN THE CYLINDER BORE AND HAVING A PISTON ROD EXTENDING THROUGH SAID CLOSURE BEARING, THE PISTON DIVIDING THE CYLINDER BORE INTO A COMPRESSION CHAMBER BETWEEN THE PISTON AND THE FOOT VALVE AND A REBOUND CHAMBER BETWEEN THE PISTON AND THE CLOSURE BEARING, BOTH OF SAID CHAMBER BEING FILLED WITH HYDRAULIC FLUID AND THE PISTON HAVING METERING ORIFICE MEANS THERETHROUGH FOR SHOCK DAMPING DISPLACEMENT OF HYDRAULIC FLUID BETWEEN SAID CHAMBERS THROUGH THE ORIFICE MEANS IN THE RECIPROCATIONS OF THE PISTON WITHIN THE CYLINDER, THE IMPROVEMENT COMPRISING: A SUBSTANTIALLY SPHERICAL FREE FLOATING MASS IN SAID COMPRESSION CHAMBER BETWEEN SAIS FOOT VALVE AND SAID PISTON AND IN UNINTERRUPTED ALIGNMENT WITH SAID ORIFICE MEANS, SAID MASS BEING OF BULK COMPRESIBLE ELASTOMERIC MATERIAL A SUBSTANTIAL PART OF THE VOLUME OF WHICH COMPRISES GAS-FILLED, SMALL, UNCONNECTED, DISCRETE AND INDEPENDENTLY ACTING, GENERALLY BUBBLE-LIKE CELLS, SAID MASS BEING OF A DIAMETER ONLY SLIGHT LESS THAN THE DIAMETER OF SAID CYLINDER BORE, WHEREBY SHOCK WAVES CREATED BY PRESSURE SURGES OF A MAGNITUDE GREATER THAN CAN BE ACCOMMODATED BY THE ORIFICE MEANS WILL BE ABSORBED BY BULK COMPRESSION OF SAID MASS AND CONTINUED COMPRESSION MOVEMENT OF THE PISTON IS REFLECTED IN RAPIDLY INCREASING SPRING RATE RESISTANCE TO COMPRESSION OF SAID MASS AND DAMPING OTHERWISE PROCEEDING BY DISPLACEMENT OF HYDRAULIC FLUID THORUGH SAID ORIFICE MEANS AND SAID FOOT VALVE. 